Re: Allowable Stress

From: <Christopher>
Date: Mon Aug 27 2001 - 11:53:00 EDT

>What is really the Code saying about this. An illustration will
>be much appreciated

In setting different limits the Code is distinguishing primary stress from secondary stress. Primary stress increases with applied loads like the hoop stress in a pipe wall, whether or not yield occurs. If a primary stress exceeds the yield stress, the vessel fails by bulging noticeably until the material work hardens, and the vessel is pretty much useless thereafter.

Secondary stress is different than primary stress because it's self limiting. An example of secondary stress is the stress that develops locally at the joint between the head of a pressure vessel and the drum. The drum wants to grow in response to internal pressure while the head doesn't grow so much. Since the pieces are actually joined, local shear and bending develop to keep the two sections attached. If the vessel starts to yield locally these loads don't increase like primary stress, because the difference in growth rates is made up by the additional growth due to yielding. Secondary stress is called self-limiting for this reason. A vessel which yields in this way develops a set of built-in stresses that eventually 'shake down' to elastic action, so the vessel's service isn't affected. The increase in allowable stress for secondary stress reflects the fact that such loading won't cause immediate failure like high primary stress.

Thermal stress has some of the characteristics of secondary and primary stress. A section of pipe that isn't allowed to expand can fail suddenly by buckling, but if it doesn't buckle the pipe yields and doesn't go anywhere. The statically indeterminate moments that develop in a pipe expansion loop are also secondary stresses, because they don't increase as the material yields.

There's a helluva fine paper that's probably still around called 'Criteria of the ASME Boiler and Pressure Vessel Code for Design by Analysis in Sections II VIII Div 2' that explains the difference in detail and includes the basis for fatigue design. (No, I don't know a web site and no, I can't fax it...) It's not for anyone who hasn't kept up with applied mechanics, but it's an excellent discussion of Code limits.

Christopher Wright P.E. |"They couldn't hit an elephant from <a href="/group/PipingDesign/post?postID=nDVLUceSLLF7ZvY6vShHvdmZ7fgknDlQulRhiYlntg2tUkBG9t-UpUMssFjmPBxfYzn1YyY4RHvw7JLNDQ">chrisw@skypoint.com</a> | this distance" (last words of Gen.

___________________________| John Sedgwick, Spotsylvania 1864)

<a href="http://www.skypoint.com/~chrisw">http://www.skypoint.com/~chrisw</a> Received on Mon Aug 27 11:53:00 2001